MELLONI ON THE NOCTURNAL COOLING OF BODIES. 533 



penetrates into the interior, and tends to fall toAvards the ground 

 in virtue of its greater specific gravity; but the mechanical 

 obstacles, and the attraction of this multitude of interlacing 

 threads, hold it suspended for some time in the neighbourhood 

 of the parts which radiate towards the sky ; there is thus pro- 

 duced a series of actions and reactions similar to that we have 

 just examined, and the mixture of air and of cotton or wool is 

 much more cooled than the simple stratum of varnish or lamp- 

 black applied to the thermometer. It is for the same reason 

 that, cateris paribus, those plants whose leaves are hairy acquire 

 a rather lower temperature than those whose leaves are smooth 

 and free from pubescence, and consequently are covered with a 

 greater quantity of dew. But to return to the meadow. In order 

 to indicate the portions of grass which are the most cooled by 

 virtue of radiation, we have just now employed constantly the 

 term "higher" instead of "summit,^' because, on examining the 

 facts with a little attention, it is quickly seen, that if the first 

 impression of greatest cold is produced at the outset at the super- 

 ficial portion of the meadow, the minimum of temperature soon 

 quits the surface, and is transferred to the interior. 



Suppose, in fact, our " lower stratum" divided into three sub- 

 divisions or elementary strata ; the first composed of the air 

 which envelopes the summit of the herbage ; the second, formed 

 of the subjacent part, where the leaves are more numerous, and 

 more or less exposed to the aspect of the zenithal region (which, 

 according to preceding observation, is the most active of all in the 

 phaenomena of nocturnal radiation) (see First Memoir) ; finally, 

 the third, composed of the air which embraces the stems and 

 leaves, entirely shut out from the aspect of the sky. The sum- 

 mits of the grass certainly are placed in the most favourable 

 conditions for radiating their heat freely into space ; but the 

 leaves are few there, and exposed to atmospheric disturbances, 

 so that the small quantity of air which is cooled by contact with 

 them scarcely produces any sensible effect on the rest of the 

 stratum. The middle portion of the meadow, being more co- 

 piously provided with leaves and more sheltered, without being 

 withdrawn from the so-powerful influence of the zenithal region, 

 still further cools the corresponding air. As to the lower por- 

 tion, which is totally shut out from the aspect of the celestial 

 vault, it can only transmit to the surrounding air the cold de- 

 rived from the communication of the stems, or from its radiation 



